/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineSessionInterface.h
Determine if the player is registered in the specified session
true if the player is registered in the session
bool IsPlayerInSession ( [FName](API\Runtime\Core\UObject\FName) SessionName, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & UniqueId )